1. Field of the Invention
This disclosure relates generally to electrical switches and in particular to a more reliable, extended travel, dome-type pushbutton switch having tactile feedback and three states of switching.
2. Description of Related Art
Tactile feedback, push button switches are well known in the art, but for certain applications the life of the switch is not as long as may be desired or necessary. Often in dome switches, the top dome gets very stressed and the switch looses its tactile feel or fails. Also, the limited movement of the activating pushbutton in three-state switches minimizes the feel of the center position.
In U.S. Pat. No. 5,172,114 issued Dec. 15, 1992 to Claude Bedoya and assigned to Sextant Avionique of France, a tactile effect switch used in a keyboard is disclosed. The tactile effect is obtained by combining a snap acting switch and the deformation of an elastomer stud, axially deformable under compression, which is integrally formed on a membrane made from a resilient material disposed on a support plate and which provides a resilient connection between a key and the switch in the manner of a pusher. A resilient blade is deformed when pressure is applied to the stud causing actuation of the switch as the blade makes contact between an inner contact and two outer contacts.
In U.S. Pat. No. 5,228,561 issued Jul. 20, 1993 to Christopher K. Schroeder et al. and assigned to Hewlett-Packard Company of Palo Alto, Calif., a long traveling pushbutton switch with enhanced user tactile feedback is disclosed. The switch comprises a keycap, a keycap plunger, a retaining bezel, an elastomeric dome switch button formed in an elastomeric sheet comprising rubber which rests on a printed circuit board having a conductive pattern. Deformation of the button switch provides tactile feedback, while impact of the keycap top against the retaining bezel provides audible feedback. The pushbutton switch is configured to limit the downward displacement of the keycap plunger to avoid excessive force on the printed circuit board.
In U.S. Pat. No. 5,199,557 issued Apr. 6, 1993 to Gert Brandt et al. and assigned to MEC A/S of Ballerup, Denmark, a key is disclosed comprising a domed metal disc with its dome facing upward, a rubber component arranged on top of the domed metal disc which comprises a membrane part and a stem part. The stem part is a hollow part, which is elastically deformable and serves the purpose of transmitting a mechanical force from a button to which the mechanical force is applied to the domed metal disc, as the metal disc is deformed or allowed to revert to its normally domed shape from its deformed shape by the elastic deformation of the stem. The key has only two states whereby the metal dome makes contact with a central contact pad when the mechanical force is applied to the button (state 1) and brakes contact when the force is removed (state 2).
In U.S. Pat. No. 5,345,051, issued Sep. 6, 1994 to Teruhisa Miike and assigned to Alps Electric Company, Ltd., of Tokyo, Japan, a pushbutton switch is disclosed which has a comparatively small height and minimal rattling of a stem upon movement. The pushbutton switch comprises an insulating case, a pair of fixed contacts on the inner bottom of the case, a moveable contact for movement toward and away from the fixed contacts a stem for operating the movable contact and being resiliently biased away from the fixed contacts, the stem having a flat plate and four legs, a click rubber element positioned below the flat plate of the stem, and guide holes formed at the four corners of the insulating case. Thus when the flat plate of the stem is depressed, the top of the click rubber element is pushed down to resiliently deform the click rubber element until the movable contact contacts the fixed contact on the inner bottom face of insulating case to short-circuit the fixed contacts. When the stem is released, the click rubber element pushes up the stem by its own resilient returning force. This switch has only two states.
In U.S. Pat. No. 5,898,147, issued Apr. 27, 1999 to Frank M. Domzalski et al., and assigned to C & K Components, Inc., of Watertown, Mass., a dual action 3 state, convex disc pushbutton switch assembly is described which provided tactile feedback to the operator. Each convex disc contact comprises four tabs, each tab of which is fitted and secured in the base of the switch assembly making the switch easy to assemble and operate reliably. The tabs of a first convex disc contact are positioned forty-five degrees relative to the tabs of a second convex disc contact. The switch is sealed thereby permitting various ways to secure the switch terminals to an electronic board. However, the top disc experiences considerable stress during its operational usage and can loose its tactile feel or fail.